Wear sunscreen EVERY DAY, regardless of the weather or whether you need to be outdoor for a long time. If you can see any daylight, UV rays are present and you need to use protection. I know it means you have to wear sunscreen every day for the rest of our lives, but it is human beings' fault in the first place to have ruined Nature's ability to shied UV rays. So we have only ourselves to blame.

A sunscreen with proper UVA protection should have at least one of the following five ingredients:

Zinc Oxide

Titanium Dioxide

Avobenzone (aka Parsol 1789 or butyl methoxydibenzoylmethane)

Mexoryl SX

Tinosorb

Do learn to check the ingredient list of the sunscreen product (or any skincare product, actually) you buy. If a sunscreen product does not have any of those ingredients, it is NOT something you should pay any money for.

(From Paula Begoun's website, Where you can find more comprehensive guide on sun protection)